This invention relates to mounting of electric strip heating elements in high temperature vacuum or protective atmosphere furnaces.
Electric heating elements in high temperature vacuum furnaces are often made of strips of sheet molybdenum or the like. Electrical contact may be made to the ends of a long metal strip which wraps around the hearth of the furnace. Such a long heating element requires mechanical support intermittently along its length to hold it in proper position in the furnace and for preventing shorting to other parts of the furnace or the load being heated in the furnace.
Various techniques for providing insulated mechanical support for such a heater element have been employed, but few are completely satisfactory. Less than satisfactory heating element supports are described and illustrated in U.S. Pat. Nos. 3,737,553 by Kreider, 3,812,276 by Cyrway and 4,056,678 by Beall, for example.
Some heater supports have relied on an insulated post, or the like, extending through a hole in the strip heater. Although good mechanical support can be provided with such an arrangement, the hole is quite undesirable. The hole necessarily results in the heater having a narrower effective width at the location of the hole. Since there is less metal cross-section to carry the heating current, there is excess heating around the hole. This excess heating can be severe enough to burn out a heater under some circumstances and, if nothing else, it shortens the heater lifetime in that region. Heater elements most commonly fail at the end electrical contacts or in proximity to such holes.
Another type of heater support that does not require holes through the heater element is in a general form of a T. A sheet metal "post" forms the leg of the T, and the heater element lies on top of the top crossbar of the T. The heater element is secured to the crossbar by a rod lying on top of the heater element with twisted wires securing the bar to the crossbar of the T. Ceramic sleeves insulate the crossbar from the leg of the T to provide a compliant connection. Supports shown in the Kreider and Cyrway patents are of this general type.
Such heater supports have been plagued with deformation problems. There is insufficient rigidity in the T-shaped mounting to support the heater element as it tries to move under the forces of thermal expansion, cooling gas flow, mechanical vibrations, and the like. The crossbar of the T tends to tilt relative to the leg, which may result in shorting of the heater element to other parts of the furnace structure or the load in the furnace. Breakage is also a problem when attempting to remove or replace the heater element supports. In high-temperature furnaces, heater elements, heat-shields and supports for the heater elements are often made of molybdenum. This material becomes quite brittle after heating to elevated temperatures.
It is also important to provide good electrical insulation between the heater element and other portions of the furnace. This electrical insulation must not only isolate the heater element when the furnace is first put into service, but must also maintain such isolation after heating. A problem encountered in high-temperature vacuum furnaces is "metallizing." Components of the furnace and articles being heated in the furnace may evolve metal vapors that deposit on electrical insulators and provide an electrically conductive path which shorts such a heater element to other parts of the furnace. The electrical insulation should resist such shorting when metallizing occurs.
The heater support must also accommodate dimensional changes in the heater and the furnace. Typically, one part of the support is at relatively low temperature, while another part is at relatively high temperature. The heater element itself undergoes thermal expansion as it is heated. The consequent dimensional changes must be accommodated by the support without applying large mechanical loads on the brittle heating element, which could result in breakage.
U.S. Pat. No. 4,771,166 has provided an excellent support for an electric heating element in a vacuum furnace. The heater support has two similar support assemblies spaced apart with a rigid bridge compliantly mounted therebetween. The heater strip is compliantly connected to the bridge. Each of the mounting assemblies has a metal post connected to the heating chamber of the furnace and surrounded by a ceramic sleeve. The ceramic sleeve is surrounded by two ceramic tubes which hold the end of the bridge.
Additional improvements could, however, be made in such a heater support for reducing heat conduction though the metal post that secures the tubes in place in the furnace chamber. This mounting support also has a number of separate pieces which must be separately fabricated and it takes appreciable time to assemble. It is desirable to provide a heater support which does not deform upon heating, supports a heater element compliantly to accommodate thermal expansion, does not penetrate the heater element and lead to localized heating, does not short out to the furnace structure due to metallizing and is made with a minimum number of parts.